A microstructure of a sample surface can be observed by irradiating a sample with electrons while scanning and by detecting secondary electron charged particles discharged from the sample. This device is referred to as “Scanning Electron Microscope” (hereinafter, referred to simply as SEM).
It has been widely known that the SEM makes it possible to obtain an observed image more easily with higher throughput in comparison with other techniques for observing a microstructure of the surface, such as, for example, an atomic force microscope or a scanning tunnel microscope. In general, an electron source for discharging electrons to be used for the SEM needs to be kept in an ultrahigh vacuum state for stabilization, and for this reason, a sample chamber for holding the sample in irradiating the sample is normally kept in a vacuum state.
For this reason, it is difficult to observe a sample whose nature changes when maintained in vacuum (for example, a biological sample and an organic sample). If a degree of vacuum is lowered in an attempt to observe such a sample, gaseous molecules undesirably flow into the electron source side through a passage (pore) for use in allowing an electron beam discharged from the electron source to pass into the sample chamber. As a result, the degree of vacuum on the electron source side is undesirably lowered.
Therefore, conventionally, as typically represented by an observation inspection for a semiconductor, the application of the SEM has been limited to a sample whose nature is not changed so much even under vacuum.
Meanwhile, along with recent increasing needs for observation of a sample surface in medical and biological fields, devices referred to as an environmental SEM and a low atmospheric pressure SEM or an atmospheric pressure SEM, have become more important.
These devices need to have a reduced vacuum conductance in a passage for allowing an electron beam to reach the sample chamber so as to make the sample chamber set to the atmospheric pressure or a pressure similar thereto, while the electron source is being kept in an ultrahigh vacuum state.
For example, Patent Document 1 discloses a device in which minute orifices with electron permeable membranes (for example, collodion membranes) are provided in a vacuum housing and differential evacuation is carried out. Moreover, an example for an electron microscope which scans a sample by moving a movable stage in place of scanning an electron beam has been described.